f label  for clause

         This item defines the start of a for loop.

         The descriptors on the top of the stack are:-
         
                                  +-----+
                                  |     |
               INITIAL VALUE      | [F] |
                                  |     |
                                  |-----|
                                  |     |
               FINAL VALUE        | [L] |
                                  |     |
                                  |-----|
                                  |     |
               INCREMENT          | [I] |
                                  |     |
                                  |-----|
                                  |     |
               CONTROL VARIABLE   | [C] |
                                  |     |
                                  |-----|
                                  |     |
                                  .     .

         'label' is the repeat label.  i.e. the label jumped
         to on repeat

         'label+1' is the exit label.  i.e. the label jumped
         to on exit or the completion of the cycle.

         'label+2' is never generated and so may be used by
         the code generator.

         e.g. @1N2N5N1f3    for J = 1,2,5 cycle

              F4            exit

              F5            continue

              :5B3:4        repeat















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